The present invention relates to a thermionic emitter intended, for example, for electronic microscopy, as well as the manufacture of such thermionic emitters. In electronic microscopy an electron source of very short wavelength is used, which is a function of the applied electrical voltage. This type of electron source must operate in a vacuum the degree of which varies according to the electron sources actually available. One of the types of electron guns, named a thermionic gun, emits electrons by heating the source incorporated in the gun. The emitted electrons are accelerated by an electric field which establishes the beam of primary electrons. The Wehnelt cylinder of the gun, maintained at a more negative potential than the cathode, focuses the emitted electrons toward the anode. The effective brightness of the gun is a function of the form and of the position of the filament with respect to the opening in the Wehnelt cylinder.
The performance of the electronic microscope is very dependant upon the gun brightness and upon the coherance and the stability of the electron source which, according to the theoretical model, must be a pin-point source. The point of operation of the filament is established by its temperature. If the filament temperature is increased the evaporation increases and the life span diminishes. In using conventional filaments, it is generally agreed that filament break down occurs when the reduction of filament diameter by evaporation attains about 6%.
Different gun brightness characteristics are obtained by varying the geometry of the filament point. The two conventional tungsten filament types are the standard model and the pointed model. The brightness of the pointed filament is markedly superior but results in an important decrease in life span (1 to 3 hours). Thermionic sources using lanthanum hexaboride provide more brightness than tungsten filaments but this improvement is leveled by increased cost (approximately 10 times more expensive). On the other hand, the life span is more acceptable. The other type of electron gun operates on the principle of electron emission in an electric field. It has superior brightness and coherance characteristics but it requires vacuum in the order of 10.sup.-9 to 10.sup.-10 mmHg resulting in increased cost of installation.
As a consequence, there is an urgent need for a tungsten filament operating on the principle of a thermionic emission having a greater life span than standard filaments (approximaxely 25 hours) and preferably offering a brightness equal to, or greater than that of the pointed filament of which the average life span is only a few hours. It must be realized that a short life span results not only in higher cost of filament replacement, but also in a sequence of fastidious operations, not to mention the inconveniences of interruption of observation work, the time consuming operations of gun cleaning; the assembly, the centering and the adjusting of the filament level in the gun, the alignment in the microscope column and finally the time required for reaching thermionic stabilization.